Multipoint field ionization source

ABSTRACT

A field ionizing source for ionizing components of a gas sample includes a multipoint array of conductive needle-like elements on a porous substrate. A grid is disposed in close proximity to the needle-like elements so that an ionizing electric field is generated between individual needle-like elements and the grid when a potential difference is established between the grid and array. The gas sample is introduced into the ionizing electric field zone by passing it through the back of the porous substrate and into the array between the needle-like elements so that substantially all of the gas is exposed to the ionizing electric field. Ions so produced are formed into an ion beam by a focusing electrode structure and directed into a spectrum analyzer and components of the gas sample are determined.

United States Patent 1191 Aberth 1 Dec. 3, 1974 MULTIPOINT FIELDIONIZATHON SOURCE [75] Inventor: William H. Aberth, Palo Alto, Calif.

[73] Assignee: Stanford Research Institute, Menlo Park, Calif.

[22] Filed: Sept. 21, 1972 [21] Appl. No.: 290,900

[52] US. Cl 250/288, 250/292, 250/423, 313/309, 313/351 [51] Int. Cl.H0lj 39/36, H0lj 1/30 [58] Field of Search 313/309, 351; 250/41.9 G,25()/41.9 SE, 41.9 SR, 282, 285, 288, 423

Primary Examiner.lames W. Lawrence Assistant ExaminerWm. H. PunterAttorney, Agent, or FirmUrban H. Faubion [5 7 ABSTRACT A field ionizingsource for ionizing components of a gas sample includes a multipointarray of conductive needle-like elements on a porous substrate. A gridis disposed in close proximity to the needle-like elements so that anionizing electric field is generated between individual needle-likeelements and the grid when a potential difference is established betweenthe grid and array. The gas sample is introduced into the ionizingelectric field zone by passing it through the back of the poroussubstrate and into the array between the needle-like elements so thatsubstantially all of the gas is exposed to the ionizing electric field.Ions so produced are formed into an ion beam by a focusing electrodestructure and directed into a spectrum analyzer and components of thegas sample are determined.

12 Claims, 4 Drawing Figures Pmmmm 3.852.595

SHEET 1 BF 2 MULTIPOINT FIELD IONIZATION SOURCE BACKGROUND OF THEINVENTION The phenomenon of ionization plays a significant role in manyscientific instruments and experiments, e.g., in ionization gauges andmass spectrometers. In mass spectrometry, an unknown material underinvestigation is ionized prior to injection into the analyzer ormass-separator section of the mass spectrometer. Ionization is usuallyproduced by electron impact with the unknown material, utilizing asuitable electron source such as a thermionic emitter. However, electronimpact with molecules not only ionizes them, but also tends to fragmentthem into two or more species, so that the mass spectrum, obtained bythis ionization method, may show the presence of the daughter speciesbut little or nothing of the parent species. Moreover, if any of thedaughter species is the same as, or has a mass-tocharge ratioapproximately equal to, another species originally present in theunknown material, then the mass spectrum obtained can be difficult orimpossible to interpret correctly regarding the original constitu tentsof the unknown material.

In some applications where mass spectrometry is used to monitor orcontrol other processes, e.g., the preparation of photoemissivesurfaces, the use of a thermionic emitter for ionization isdisadvantageous because the heat or light from the emitter tends todisturb the process. The use of a cold nonluminous ionizer in suchapplications constitutes a significant improvement. Field ionization, aphenomenon in'which molecules entering a region of very high electricfield to 10 V/cm) are ionized by extraction of electrons by the field,causes substantially less fragmentation than electron-impact ionization..Also, this phenomenon does not require or involve the generation oflight or heat. I

ln order to reduce to a practical level the voltage required forproducing the required high fields, sharp blades, needles or points areused as field ionizing electrodes, a counterelectrode is spaced from theneedlelike structures and a voltage is applied so that thecounterelectrode is negative relative to the blades or needlelikestructures. However, even with the use of sharp points, if thecounterelectrode is spaced a macroscopic distance from the blades orpoints, e.g., of the order of millimeters, the voltages required for thefield ionization are of the order of tens of kilovolts.

Ionization efficiency of prior art field ionizers of the single blade orsingle needle-like structure is low since the effective region whereionization takes place is confined to the small volume in the immediatevicinity of the apex of the sharp point or blade edge, so that the rateof ion production for a given pressure of material to be analyzed ismuch lower for field ionization than for electron-impact ionization.Another reason is that the field-produced ions attain velocitiesequivalent to the voltage applied 'between'ionizer and counterelectrodeand the ions are impelled away from the ionizer over a wide range ofangles, so that only a small fraction of the ions is collimated into abeam suitable for injection into the analyzer of the mass spectrometerwithout employing complex ion-optical lenses.

Parallel operation of many needle-like members to provide acorrespondingly large ionization volume is feasible, but the problems offormation of the parallel structures and providing ion-opticalcollimation are formidable. For example, ion-optical collimation ispractical only if emission energies of the ions can be kept small, whichnecessitates spacings between the ionizer and counterelectrode of theorder of microns with the ionizer point having a tip radius of afraction of a micron, e.g., 0.1 micron. Also, it is desirable to spacethe needle-like structures as close together as possible withoutincurring significant decrease of the field at each point by thepresence of its neighbors.

Many of the problems involved in the construction of arrays of the fineneedle-like structures and the problems thought to be inherent inparallel operation of such structures have been solved by structures andmethods of producing the structures as disclosed in US. Pat. No.3,453,478, entitled Needle-Type Electron Source, issued July 1, 1969,and US. Pat. No. 3,497,929, entitled Method of Making a Needle-TypeElectron Source, issued Mar. 3, 1970, to Kenneth R. Shoulders and LouisN. l-Ieynick. Further refinements are illustrated and described in US.Pat. No. 3,665,241, entitled Field Ionizer and Field Emission CathodeStructures and Methods of Production, issued May 23, 1972, to Charles A.Spindt and Louis N. Heynick. The subject matter of these patents isspecifically incorporated by reference.

A highly refined and practical field ionization source is provided usingthe teachings of the above patents. A bare-point structure is providedin which a regular array of closely spaced metallic points of controlledgeometry is provided over the surface of a conductive substrate and ascreen-like counterelectrode is placed in close proximity to the pointsof the needle-like elements with apertures in the screen-likecounterelectrode in register with the points. A field ionizationstructure is provided by making the counterelectrode of the arrangementjust described negative relative to the substrate electrode andproviding the proper electrode counterelectrode spacing as well as ratioof such space to the distance between electrode points. The sample to beionized is introduced in the area of the needle-like structures.

While the field ionization just described is highly efficient relativeto prior art field ionization sources, it is still desirable to increasethe ratio of sample particles ionized to sample particles introduced,i.e., increase the efficiency of ionization. This is especially criticalin the many applications where the total sample of the material to beanalyzed is very small.

SUMMARY OF THE INVENTION In carrying out the present invention, a fieldionization source for ionizing components of a gas sample is provided byutilizing ahighly regular array of closely spaced needle-like points ofcontrolled geometry on the surface of a porous substrate. An electricfield is provided in the area of needle-like elements by disposing aconducting grid in close proximity of the needlelike elements withapertures in the grid in register with the points of the needle-likeelements and providing a potential difference between the grid andneedle-like elements. The gas sample to be ionized is introduced intothe ionizing electric field zone by passing it through the poroussubstrate and into the ionizing electric field around the needle-likeelements so that substantially all of the gas passes in close proximityto the descrip tiontal ten in connection with the accompanying drawings.7

BRIEF-DESCRIPTION OF DRAWINGS FIG. 11 is an enlarged perspective viewshowing a bare-point array'utilized in the field ionization source ofone embodiment of the invention; FIG. 2 is an enlarged'fragmentaryprospective view at abare-point array using a different substratearrangement (different from the one illustrated in FIG. 1) for oneembodiment of the field ionization source;

' FIG. 3 is a central, vertical, sectional view of a field ionizationsource structure utilizing the bare-point .array of HG. '1; and

, FIG, 4 is a diagrammatic cross-sectional view-ofa mass spectrometerutilizing the field ionization source.

DESCRIPTION QOFYPREFERRIED EMBODlMENTS I l-and the array ofneedle-like'elements 22 which are used to form the ionizing electricfield are laid down on the conductive coat 2i) on lands defined by thesubstrate surface areabetween the open channels 18. The array ofneedle-like elements 22 and 12in the embodiments of both FIGS, land 2may be resistive, semiconductive, insulative or composite materials andtheir surfaces overcoated or otherwise treated to obtain the desiredcharacteristics just as taught in the patents previously referred to.Further, the arrays of needle-like elements may be formed as describedinvthose patents.

The disk-shaped glass substrates are'available commercially from BendixCorporation and Varian Associates and are used for other purposes. Theplate consists of microscopic hollow glass channels-fused into adiskshaped array. The disk is practically obtainable in sizes ranging upto 25 millimeters in diameter (a diameter about equivalent to that of aquarter), about 5 millimeters thick (about one third the'thickness of aquarter),

v I A form of thebasic bare'-pointarray 10 used as a field. I

ionization source is illustrated in F 1G,]. The bare-point arraystructure-l0 includes a supporting disk-like sub.- strate 11 and anarray of electric field-forming needlelike bare points 12 formedthereon. Since an important consideration in the design of the sourceisto provide a way to bring the sample to be ionizeddirectly into theionizing electric field for most efficient ionization, the substrate ismade porous so, that the sample can be brought in through" the back ofthe'substrate and up along the surface of the field-forming needlel ikeelements 12. As illustrated, the bare points are pyramidal but' may beof other conical shapes. The substrate 11 in'this embodiment'is ofporous sintered tungsten which is conductive" and," therefore along withthe tines 'a,,conductive. electrode. In one-embodiment, the tungstensubstrate 11 is'65 percent porous and the neeg dle-like points1 2 covera circular area 1.5 millimeters in diameter and are spaced 0.0025centimeter apart (center-to-center). I For some applications thepermeability of the porous tungsten substrate 11 may not be sufficientor it may not have the characteristics desired. For example, thoughtungstenis considered inert, it may, in fact, chemically'affectparticular samples; therefore, it may be preferable to use asemiconductive or an insulative substrate. One such embodiment isillustrated in FIG. .2; The substrate illustrated in FIG. 2 constitutesa glass disk 16 which has a myriad of open channels 18 all of the waythrough the disk. A practical disk substrate 16 for the arrayis about1.5 millimeters in diameter with about-60 percent of the disk comprisingstraight, .open channels.-

array of needle-like points 12formed thereon, consti and containsapproximately 1,670,000 open channels. Thus about 60 percent of the diskis comprised of open channels. Suchdisks are shown and described in thecatalog entitled Application for Microchannel Plates,.publishedby'Varian, Palo Alto Tube Division. The surface of the microchannelplates, including the interior of the channels as described in thecatalog, is semiconductive and the platesdescribed are entirely suitableas substrates for the field-forming arrays for the present fieldionization source. Other practical insulative-substrates are madeofsintered glass and porous alumina. 5

A setting for the bare point. array 10 of FIG. 1 is illustrated in FIG..3.'.The"field ionization source 30 .illus trated in FlG. 3 is designedspecifically'to provide a convenient way to direct theflow of a gassample throughjthe back of the porous sintered tungsten sub- 'strate' 11and around the electrical field-forming f needle-like elements 12..Thefield'ionization source 30,

illustrated, also provides means for providing-electrical connectionstoe'le'ment's of the source in order to generate the ionizing field. The.gas flow channel 32 to the back of the sintered plug 11 is identifiedby a series of arrows which extend directly through the device.Electri'cal connections must be made to the sintered tungsten plug 11and a field-forming grid structure 34 spaced from and parallel to the"plane of the ionizing points on the needle-like field ionizer 12. lConsider first the physical mounting structure for the multipointionizing array 10 which also serves as the conductive electricalconnection thereto. The source- 30 is constructed so that it is easilyremovable in order to be able to replace a damaged or inactivemultipoint array 1.0. The sintered tungstenplug 11, of the array 10 ismounted at one end I'( right in the figure) of a tubular conductivearray support member-3621's by brazing or" weldingtln order to provide ameans for removably mounting the support tube 36 to 'a'supportingconduc- Thus,it is seen that the substrate is highly porousand showslittle resistance to gasflow-through. in order to provide a conductiveelec trode, the upper surface of the substrate 14 is provided with a:layer of conductive or semiconductive material 20 whichdoes not cover(doesnot close) the openichannels IS in the substrate tive washer orcollar 38, the tubular member 36 is threaded internally at its opposite(left) end (at 37) so as to receive the external threads of a bolt 40which ex-' tends up through a centrally located threaded aperture 39 inthe support collar 38 and also threads into the internal threads 37 ofthe support tube 36. The collar 38 is also provided with a recess 41 toreceivethe head of the securing bolt 40. in order to provide a clearopen channel for gas flow to the'back of the sinte red tungsten plug 11,a bore or channel.4 2' isjprovide'd through the securing bolt 40. Thestructure thus far described is rigidly held together as a unit, iselectrically conductive so that the electrical connections can be madeto the multipoint field ionizer array 10, and has a clear opening to theback of the multipoint array so that a gas sample may be introduced.

in order to secure the multipoint field ionizer assembly, justdescribed, to the instrument in which it is to be used, here a massspectrometer, the instrument itself is provided with a highly conductiveheat sink and ion source support 46 which is illustrated as a heavycylindrical copper bar 46 provided with a centrally located longitudinalaperture, that forms part of the gas flow channel 32, and externalthreads around the upper end. The field ionizer assembly is then securedto the rodlike heat sink 46 by an internally threaded conductive cap 48that threads on to the outer end (threads 47 of the heat sink 46 and hasan inwardly extending annular lip 49 that fits snugly into an annularrecess 50 around the outer periphery of the collar 38 of the fieldionizer assembly. In order to provide a good electrical and thermalconduction between the heat sink 46 and the field ionizer assemblymounted thereon, washer 52 of relatively soft, highly conductivematerial is provided between the upper surface of the heat sink 46 andthe lower surface of the field ionization source supporting washer 38.Gold is a highly acceptable material for washer 52.

Thus, it is seen that a clear gas flow channel 32 is provided throughthe heat sink 46, the supporting bolt 40 and tubular multipoint fieldionizer array support member 36 to the back of the porous sinteredtungsten plug 1 1. Further, all of the parts thus described areconductive of both heat. and electrical current. Thus a common pathconductive of both heat and electricity is provided for the multipointfield ionizing array 10. Since a potential difference must beestablished between the grid 34 of the ionization source and theneedle-like points 12 of the array 10, a tubular insulating grid supportmember 54 is mounted on the upper surface of the field ionizationassembly support collar 38 so that it surrounds the support tube 36 ofthe multipoint array 10 and is concentrically spaced therefrom. Thelength of the insulating tubing member 54 is such that the grid- 34 isspaced above the plane of the ionizing points of the needle-likeelements 12 by the appropriate distance.

in the source illustrated here, the grid 34 is a mesh of 400 lines percentimeten'is spaced about 0.0125 centimeter above the points with theopenings therein positioned in register with the points of theneedle-like elements 12. The grid 34 is firmly secured in place on theupper end of the insulating tube 54 by a plug-like stainless steel cap56 which is open in the center to allow egress of ions formed by thesource. Retaining cap 56 is provided with appropriate threaded screwrecesses 58 extending through its upper surface and screw recess 59which extends through its outer periphery. Set screws 60 are provided inthe screw recesses to 58 and 59 to engage the upper end and outerperiphery, respectively, of the supporting insulating tube 54 to holdthe cap 56 firmly in position.

The insulating tube 54 is firmly secured to the upper surface of thesupport collar 38 of the field ionization source by a clamp arrangement.Specifically, an annular retaining groove or recess 51 is providedaround the periphery of the grid support member 54 near the end oppositethe grid 34. A hold down collar or washer 53 is positioned in theretaining groove 51. The grid assembly hold down arrangement iscompleted by hold down bolts 55 that pass through apertures in the holddown collar 53 and are threaded into internally threaded apertures 35provided in the upper surface (toward the grid 34) of the sourcesupporting washer 38. Thus, the grid 34 is properly positioned relativeto the-multipoint elements 12 and held firmly in place.

The field ionization source 30 is shown, diagrammatically, in a massspectrometer (the setting where it is used) in FIG. 4. Since the massspectrometer, aside from the novel ion source and the novel combination,is conventional, it is illustrated only diagrammatically and not ingreat detail. A vacuum-tight envelope is provided to enclose the entiremass spectrometer. The multipoint field ion source is illustrated in oneend immediately followed by conventional focusing electrodes 62 whichcollimate the ions developed by the multipoint ion source 30 into astream and direct the stream into a conventional quadrupole analyzersection 64. A conventional ion collector 77 is provided at the oppositeend of the mass spectrometer to receive transmitted ions. As is usual, avacuum pump 68 is connected to maintain the proper vacuum in thespectrometer envelope 60.

For operation in the intended manner, the gas flow channel 32 from theback of the porous tungsten plug 11 of the multipoint field ionizerarray 10 may be traced back through a supply line 70 to a source ofsample gas 72. The gas-flow entry channel 32 is provided with an on-offvalve 74 so that the channel may be closed entirely and also a variableleak valve 76 for fine control of the sample admitted for ionization.For normal field ionization operation, a point to grid potentialdifference of 3,645 volts is applied. The quadrupole section used haspole pieces 1.6 centimeters in'diameter and 22 centimeters long. Themultipoint source 30 is maintained at +45 volts and the extraction grid34 kept at -3,600 volts. The ions leaving the grid are decelerated andfocused by a single aperture electron optical lens 62 with a potentialof about -400 volts. The field produced ions enter quadrupole ionizersection 64 through a 5 millimeter aperture with a net energy of 45 ev.Using toluene as sample gas, an ionization efficiency of about 1 in3,000 and a transmision efficiency of l in 330 is obtained. Thus, forevery million sample molecules of toluene, one is ionized, massedanalyzed, and detected.

In order to obtain a comparison of results between ionization using themultipoint field ion source 30, as just described, and using the samesource bringing the sample into the area of the multiple points from theside or top rather than through the porous sintered tungsten plug 11, asecond supply line 80 is connected to the sample source 72 just prior toan on-off valve 82 in the regular gas flow channel 32. Thesecond supplyline 80 is connected to provide entry of sample gas into the massspectrometer envelope 60 along the side in the region of the multipointarray 10. The auxiliary supply line 80 is also provided with an on-offvalve 84 to allow the line to be opened and closed. By closing onoffvalve 82in the normal supply line 70 and opening the normally closedvalve 84 in auxiliary supply line 80, the sample effluent from variableleak valve 74 is diverted directly into the spectrometer envelope 60.The signal obtained with the same sample flow using-the limited to thespecific structures since many modifications may be made both in thematerial and the arrangement of elements. It is contemplated that theappended claims will coversuch modifications which fall within the truespirit and scope of the invention.

What is claimed is:

l. A field ionization source comprising a plate-like porous substratepervious to flow of substantially all gases and a multiplicity ofneedle-like elements located on one surface of said substrate, saidneedle-like elements being highly uniform in space and uniformly spacedon said substrate.

2. A field ionization source, as defined in claim 1, wherein both saidsubstratesurface on which the needle-like elements are deposited andsaid needle-like elements consist of conductive material;

3. A field ionization source comprising a plate-like porous substratepervious to flow of substantially all gases and a multiplicity ofneedle-like elements located on one surface of said substrate, saidsubstrate having uniformly spaced land areas thereon defined byuniformly spaced, apertures therethrough whichprovide the porosity, saidneedle-like elements being highly uniform in space, uniformly spacedonsaid substrate, comprised of conductive material and located on the saidland areas of said substrate.

4. A field ionization source, asdefined in claim 3, wherein saidplate-like substrate is of a substantially nonconduct'ive material witha plate-like conductive electrode at least on one surface thereof, saidplate-like electrode having aperture therein in registry with theapertures in said'nonconductive substrate and said conductive needleslocated on said conductive electrode.

5. A field ionization source for ionizing components of sample gases,including the structure defined in claim 1, and means to introduce asample gas to be analyzed to the surface of said plate-like substrateopposite that occupied by said needle-like elements whereby sample gasflow is provided through said porous substrate and around saidneedle-like elements, grid means spaced from said needle-like elementsand means for applying a potential between said grid and said needlelikeelements thereby to provide an electric field therebetween whereby thesaid gas sample is exposed to the said electric field in the area ofsaid needle-like elements and molecules of said gas sample are therebyionized.

6'. A field ionization source for ionizing components of sample gases,including the structure defined in claim 2, and means to introduce asample gas to be analyzed to the surface of said plate-like substrateopposite that occupied by said needle-like elements whereby sample gasflow is provided through said porous substrate and around saidneedle-like elements, grid means spaced from said needle-like elementsand means for applying a potential between said grid and said needlelikeelements thereby to provide an electric field therebetween wherebysubstantially all of the said gas sample is exposed to the said electricfield in the area of said needle-like elements.

7. A field ionization source for ionizing components of sample gases,including the structure defined in claim 3, and means to introduce asample gas to be analyzed to the surface of said plate-like substrateopposite that occupied by said needle-like elements whereby sample gasflow is provided through said porous substrate and around saidneedle-like elements, grid means spaced from said needle-like elementsand means for applying a potential between said grid and said needlelikeelements thereby to provide an electric field therebetween whereby thesaid gas sample is substantially all exposed to the said electric fieldin the area of said needle-like elements.

8. A field ionization source for ionizing components of sample gases,including the structure defined in claim 4, and means to introduce asample gas to be analyzed to the surface of said plate-like substrateopposite that occupied by said needle-like elements whereby sample gasflow is provided through said porous substrate and around saidneedle-like elements, grid means spaced from said needle-like elementsand means for applying a potential between said grid and said needlelikeelements thereby to provide an electric field therebetween whereby thesaid gas sample is substantially all exposed to the said electric fieldin the area of said needle-like elements.

9. A mass spectrometer, including the structure as defined in claim 5,including means to focus said ions into an ion beam and means to analyzeions in said beams whereby constituent components of said sample gas aredetermined.

10. A mass spectrometer, including the structure as defined in claim'6,including means to focus said ions into an ion beam and means to analyzeions in said beams whereby constituent components of said sample gas aredetermined.

11. A mass spectrometer, including the structure as defined in claim 7,including means to focus said ions into an ion beam and means to analyzeions in said beams whereby constituent components of said sample gas aredetermined.

12. Amass spectrometer, including the structure as defined in claim 8,including means to focus said ions into an ion beam and means to analyzeions in said beams whereby constituent components of said sample gasaredetermined.

ll l 6

1. A field ionization source comprising a plate-like porous substratepervious to flow of substantially all gases and a multiplicity ofneedle-like elements located on one surface of said substrate, saidneedle-like elements being highly uniform in space and uniformly spacedon said substrate.
 2. A field ionization source, as defined in claim 1,wherein both said substrate surface on which the needle-like elementsare deposited and said needle-like elements consist of conductivematerial.
 3. A field ionization source comprising a plate-like poroussubstrate pervious to flow of substantially all gases and a multiplicityof needle-like elements located on one surface of said substrate, saidsubstrate having uniformly spaced land areas thereon defined byuniformly spaced apertures therethrough which provide the porosity, saidneedle-like elements being highly uniform in space, uniformly spaced oNsaid substrate, comprised of conductive material and located on the saidland areas of said substrate.
 4. A field ionization source, as definedin claim 3, wherein said plate-like substrate is of a substantiallynonconductive material with a plate-like conductive electrode at leaston one surface thereof, said plate-like electrode having aperturetherein in registry with the apertures in said nonconductive substrateand said conductive needles located on said conductive electrode.
 5. Afield ionization source for ionizing components of sample gases,including the structure defined in claim 1, and means to introduce asample gas to be analyzed to the surface of said plate-like substrateopposite that occupied by said needle-like elements whereby sample gasflow is provided through said porous substrate and around saidneedle-like elements, grid means spaced from said needle-like elementsand means for applying a potential between said grid and saidneedle-like elements thereby to provide an electric field therebetweenwhereby the said gas sample is exposed to the said electric field in thearea of said needle-like elements and molecules of said gas sample arethereby ionized.
 6. A field ionization source for ionizing components ofsample gases, including the structure defined in claim 2, and means tointroduce a sample gas to be analyzed to the surface of said plate-likesubstrate opposite that occupied by said needle-like elements wherebysample gas flow is provided through said porous substrate and aroundsaid needle-like elements, grid means spaced from said needle-likeelements and means for applying a potential between said grid and saidneedle-like elements thereby to provide an electric field therebetweenwhereby substantially all of the said gas sample is exposed to the saidelectric field in the area of said needle-like elements.
 7. A fieldionization source for ionizing components of sample gases, including thestructure defined in claim 3, and means to introduce a sample gas to beanalyzed to the surface of said plate-like substrate opposite thatoccupied by said needle-like elements whereby sample gas flow isprovided through said porous substrate and around said needle-likeelements, grid means spaced from said needle-like elements and means forapplying a potential between said grid and said needle-like elementsthereby to provide an electric field therebetween whereby the said gassample is substantially all exposed to the said electric field in thearea of said needle-like elements.
 8. A field ionization source forionizing components of sample gases, including the structure defined inclaim 4, and means to introduce a sample gas to be analyzed to thesurface of said plate-like substrate opposite that occupied by saidneedle-like elements whereby sample gas flow is provided through saidporous substrate and around said needle-like elements, grid means spacedfrom said needle-like elements and means for applying a potentialbetween said grid and said needle-like elements thereby to provide anelectric field therebetween whereby the said gas sample is substantiallyall exposed to the said electric field in the area of said needle-likeelements.
 9. A mass spectrometer, including the structure as defined inclaim 5, including means to focus said ions into an ion beam and meansto analyze ions in said beams whereby constituent components of saidsample gas are determined.
 10. A mass spectrometer, including thestructure as defined in claim 6, including means to focus said ions intoan ion beam and means to analyze ions in said beams whereby constituentcomponents of said sample gas are determined.
 11. A mass spectrometer,including the structure as defined in claim 7, including means to focussaid ions into an ion beam and means to analyze ions in said beamswhereby constituent components of said sample gas are determined.
 12. Amass spectrometer, including the structure as defined in claim 8,including means to focus said ions into an ion beaM and means to analyzeions in said beams whereby constituent components of said sample gas aredetermined.